题目内容
（请给出正确答案）
The sonar devices must be placed ______.
A．before the undersea vehicle dives
B．at the time when the undersea vehicle is diving
C．from time to time
D．after the undersea vehicle dives
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更多“The sonar devices must be plac…”相关的问题
The sonar devices must be placed ______.
A．from time to time
B．after the undersea vehicle dives
C．at the time when the undersea vehicle is diving
D．before the undersea vehicle dives
Manned submersibles.(潜水器), like spaceships, must maintain living conditions in an unnatural environment. But while a spaceship must simply be sealed against the vacuum of space, a submersible must be able to bear extreme pressure if it is not to break up in deep water.
In exploring space, unmanned vehicles were employed before astronauts. In undersea exploration, on the other haled, men paved (铺路) the way, and only recently have unmanned remote-operated vehicles (ROVs) been put to use.
One reason for this is that communicating with vehicles in orbit is much easier than talking to those underwater. A vacuum is an ideal medium for radio communications, but underwater communications are limited to much slower sound waves. Thus, most undersea vehicles—particularly ROVs—operate. at the end of long process.
For a similar reason, knowing where you are undersea is much more difficult than in space. A spaceship's position can be located by following its radio signal, or by using telescopes and radar. For an undersea vehicle, however, a special network of sonar (声纳) devices most be laid out in advance on the ocean floor in the area of a dive to locate the vehicle's position.
Though undersea exploration is more challenging than outer space in a number of respects, it has a distinct advantage: Going to the ocean doesn't require the power necessary to escape Earth's gravity. Thus, it remains far less expensive.
People did not begin to use unmanned vehicles in undersea exploration until recently because of ______.
A．the ocean depths
B．the communication problem
C．the movement of waves
D．the problem of vacuum
Smart Cars, Intelligent Highways
Smart features of cars today
Cars today are smart. No, they may not be smart enough to change their own oil or find the lost coins in their seats, but they are smart and getting smarter. The average car today has more computing power than the 1969 Apollo 11 spacecraft that carried the first astronauts to the moon. Every car produced today has at least one computer for monitoring fuel consumption and pollution controls, The average car uses twelve computerized devices, and high-end cars have many more, controlling everything from the sunroof to the braking system. In the near future, cars may be virtually stuffed with computer chips from front fender to taillight. That's because motorists enjoy computerized gadgets(器具), and providing these little devices is cheaper for automakers than building a better engine or making other engineering changes that might actually be more important.
Many of the smart features we are seeing today are safety-related. Some are systems to avoid collisions. These may use sonar, radar, lasers, computers, or video cameras, or some combination of these. These systems beep or warn drivers with a voice signal if the vehicle gets too close to an object or another vehicle or if it strays out of its lane. The system can suggest actions to the driver or even temporarily take control to avoid accidents. Another safety device is a smart airbag system. To deploy airbags with the minimum necessary force, sensors determine an occupant's weight and size and the severity of impact. This system should reduce the number of children hurt by airbags that open too vigorously. Another system can automatically notify emergency services that an accident has happened and, using a Global Positioning System (GPS), can pinpoint the location of the vehicle for police and rescue units. This "mayday(呼救信号)system" can save precious minutes and many lives.
One of the most convenient aspects of smart cars is their ability to navigate. Drivers tell them where they want to go and then, by means of a GPS navigation device and computerized maps, smart cars can figure out the best ways to reach the drivers' destinations. The cars can show the information on a map or give drivers voice directions. They can even correct drivers if they make a mistake ("I TOLD you to turn LEFT back there; you idiot!"). Using communication devices connected to the Internet, cars can inform. drivers of problems ahead construction work, traffic jams, and accidents — and then suggest different routes to the drivers' offices, favorite pizza places, or closest shopping malls.
Problems existed
Smart cars create problems as well, however. One problem is how to control all this automotive technology. More buttons take more of the drivers' attention. Even voice controls are distracting for drivers. A recent study showed that drivers talking on handheld cell phones were four times more likely to be involved in accidents as drivers who were not. In fact, drivers using cell phones were almost as likely to be involved in accidents as those who were legally intoxicated(醉酒的). Using voice controls, even a hands-free system, might prove to be as distracting as chatting on the phone. Nevertheless, the auto industry's answer to the control problem so far has been voice control. When it comes to simple tasks -- changing channels on the radio or opening the trunk -- voice controls work well enough. But it is probably not the best method for directing more difficult operations such as navigating the Internet or controlling the car itself. Engine noise, highway noise, and the music on the stereo tend to garble(扭曲) instructions, and voice recognition systems often cannot decipher(识别) strong accents.
Intelligent roadways — a potential solution？
No matter how smart cars become, they cannot solve all the problems facing a "car-crazy" w
A．Y
B．N
C．NG
The mosquito is under study because its sonar is much more efficient than man-made sonar.
A．Y
B．N
C．NG
Which of the following would most likely be the topic of the next paragraph 1？
A．Other uses of radar.
B．Uses of sonar technology.
C．Other technology used by pilots.
D．A history of flying.
century by the Italian naturalist Lazzaro Spallanzani.He caught some bats in a bell tower，blinded them，and released them outdoors.Four of these blind bats were recaptured after they had found their way back to the bell tower，and on examining heir stomachs‘ contents，Spallanzani found that
they had been able to capture and gorge themselves with flying insects.We know from experiments that bats easily find insects in the dark of night ， even when the insects emit no sound that can be heard by human ears.A bat will catch hundreds of soft-bodied，silent-flying moths or gnats in a single hour.It will even detect and chase pebbles or cotton spitballs tossed into the air.
16.According to the author，the sonar system of bats is an example of the idea that.()
A.this is the age of technological triumphs
B.modern machines are inefficient
C.living mechanisms are often more efficient than man-made
machines
D.artificial imitations are always less efficient than living
mechanisms
17.The author suggests that the sonar system of bats ().
A.was at the height of its perfection 50 million years ago
B.is better than man-made sonar because it has had 50 million years to be refined
C.would have been discovered by man many years ago
D.is the same as it was 50 million years ago
18.The fact that“blind bats”will detect and chase cotton spitballs as well as insects is remarkable because.()
A.bats do not eat spitballs
B.cotton is harder to track
C.Spitballs make no sounds audible to human ears
D.there is purpose in the flight of insects
19.This passage was written to illustrate().
A.the deficiencies of man-made sonar
B.the dependence of man upon animals
C.that we are living in a machine age
D.that the sonar system of bats is remarkable
20.Which of the following is the main point of the passage？()
A.A bat will catch hundreds of gnats in a single hour
B.Here is a perfection in nature which sometimes cannot be matched by man‘s creative efforts
C.The phrase“blind as bat”is valid
D.director of NIH learned of the abuse
